This invention relates to an improvement applicable to a switching power supply and to an adapter employable for a switching power supply. More specifically, this invention relates to an improvement brought out for an object to enable the switching power supply to continue supplying electric power for a certain period, despite supply of the input power is suspended and to an adapter to be adapted to a switching power supply available in the prior art to enable the switching power supply to continue supplying electric power for a certain period, despite supply of the input power is suspended.
A switching power supply which is employed for supplying electric power to some of the electronic equipment employable for industrial purposes is required to continue supplying electric power for a certain period, even after supply of the input power is suspended (Hereinafter referred to as a power failure.). Otherwise, some of the electronic equipment employable for industrial purposes such as numerical control equipment (Hereinafter referred to as NC equipment.) controlling a machine tool or the like has a possibility to erroneously work, potentially resulting in breakage of the machine tool or the like controlled thereby. For the purpose to avoid these accidents, it is required to promptly detect a power failure and to allow the NC equipment or the like to take a sequential procedure to stop the function thereof without being accompanied by any malfunction, within the period in which the switching power supply keeps supplying electric power. The voltage required for the switching power supply to keep supplying during the foregoing period, however, is not the normal voltage but the minimum voltage required to allow the NC equipment or the like to conduct the foregoing limited procedure to stop the function thereof.
In this specification, the interval between a power failure and the detection thereof is called a time length for detecting a power failure, and the interval between the detection of a power failure and the time at which the voltage of the switching power supply has decreased to the foregoing minimum voltage is called a time length wherein the DC output voltage V.sub.O is held.
Available in the prior art are two exemplary switching electric power supplies which are able to keep supplying electric power for a certain period, even after supply of the input electric power is suspended.
A first example of a prior art switching power supply having the ability to continue supplying electric power after a power failure will not be discussed.
This switching power supply includes a rectifier and a switching circuit to intermit the DC current, wherein a means for detecting a power failure is attached to the AC circuit.
Referring to FIG. 1 the switching power supply in accordance with the first example available in the prior art is provided with a rectifier 40 which rectifies an AC voltage V.sub.I into a DC voltage, a capacitor 1, a switching means 2, a transformer 3, a diode 4, a flywheel diode 5, an inductance 6, a smoothing capacitor 7, a first voltage comparator 8 which compares the DC output voltage V.sub.O and a reference voltage V.sub.R a pulse width modulation means 9, and a means for detecting a power failure 10.
The DC current flowing out of the rectifier 40 is charged into the capacitor 1. The switching means 2 is closed during a period .DELTA.T out of a predetermined clock period T. The ratio of the period .DELTA.T and the clock period T (Hereinafter referred to as a duty ratio .DELTA.T/T) is automatically determined by a pulse width modulation means 9 referred to later, to be suitable to maintain the DC output voltage V.sub.O at a predetermined value, regardless of variation in the AC input voltage V.sub.I and variation in a load of the switching power supply. In response to the intermittent current flowing in the primary winding 31 of the transformer 3, an intermittent voltage is induced in the secondary winding 32 of the transformer 3. This intermittent voltage supplies electric power out of the switching power supply to the load through the diode 4, the inductance 6 and the smoothing capacitor 7. During the period .DELTA.T in which the switching means 2 is closed, the electric power is directly supplied to the load and during the period (T.DELTA.T) in which the switching means 2 is open, the electric power stored in the inductance 6 is discharged therefrom and is supplied to the load through a flywheel diode 5 and the smoothing capacitor 6. Due to the function of the smoothing capacitor 6, the output voltage V.sub.O becomes a smooth DC voltage.
The DC output voltage V.sub.O is always monitored, and the first voltage comparator 8 compares the DC output voltage V.sub.O and the reference voltage V.sub.R. In response to the voltage difference .DELTA.V, the pulse width modulation means 9 functions to determine the duty ratio .DELTA.T/T or the ratio of the period .DELTA.T in which the switching means 2 is closed and the clock period T. A signal representing the duty ratio .DELTA.T/T is inputted to the switching means 2 for the ultimate purpose to maintain the DC output voltage V.sub.O at a constant value.
Since the means for detecting a power failure 10 is connected to the AC power supply which supplies electric power to the rectifier 40, a power failure can be detected very promptly or within a time length for detecting a power failure illustrated by (t.sub.1 -t.sub.0) in FIG. 2 (d). This power failure signal is immediately inputted to NC equipment or the like which is supplied electric power by the switching power supply. Therefore, the NC equipment or the like is allowed to immediately begin the sequential procedure for stop the NC equipment or the like without being accompanied by any malfunction.
Referring to FIG. 2 and the time chart, shown described below will be the behavior of the switching power supply of the foregoing first prior art example, in case of a power failure.
As is illustrated in FIG. 2 (a), an AC power supply is supposed to suspend supply of electric power at the time t.sub.0. As is illustrated in FIG. 2 (b), the voltage of the capacitor 1 gradually decreases. As a result, the DC output voltage V.sub.O begins to decrease. The first voltage comparator 8, however, detects the decrease in the DC output voltage V.sub.O and inputs the voltage difference .DELTA.V (the difference between the DC output voltage V.sub.O and the reference voltage V.sub.R) to the pulse width modulation means 9. In response to this voltage difference .DELTA.V, the pulse width modulation means 9 increases the duty ratio .DELTA.T/T. As a result, the switching means 2 increases the time length in which the switching means is closed. In this manner, the DC output voltage V.sub.O is held at the reference voltage V.sub.R until the time which is illustrated by t.sub.2 in FIG. 2 (c) and at which the duty ratio arrives at the predetermined maximum value. Since the switching means 2 is not allowed to increase the time length in which the switching means 2 is closed any more, however, the DC output voltage V.sub.O begins to decrease at the time t.sub.2, as is illustrated in FIG. 2 (C), and it continues decreasing beyond the acceptable minimum voltage (the minimum voltage with which NC equipment or the like is allowed to stop the function thereof without being accompanied by any malfunction ) at the time t.sub.3 illustrated in FIG. 2 (c). It is not sure if the NC equipment or the like continues functioning normally after the time t.sub.3.
On the other hand, the means for detecting a power failure 10 detects a power failure immediately after occurrence of the power failure or at the time t.sub.1 illustrated in FIG. 2 (d), and the signal is immediately inputted to NC equipment or the like. Therefore, the NC equipment or the like is allowed to begin the sequential procedure to stop the function thereof at the time t.sub.1. Therefore, for the purpose to prevent NC equipment or the like from making any accidental behavior, upon a power failure, the time length (t.sub.3 -t.sub.1) between the time t.sub.1 at which the power failure is detected and the time t.sub.3 at which the DC output voltage V.sub.O has decreased to the acceptable minimum voltage, is required to be longer than the time length required for NC equipment or the like to finish the procedure for bringing the equipment to a stop.
The time length (t.sub.3 -t.sub.1) between the time t.sub.1 at which the power failure is detected and the time t.sub.3 at which the DC output voltage V.sub.O has decreased to the acceptable minimum voltage is the time length wherein the DC output voltage V.sub.O is held referred to above. The electrostatic capacity of the capacitor 1 included in the switching power supply is required to be selected to large enough to make the time length wherein the DC output voltage V.sub.O is held longer than the time length required for NC equipment or the like to finish the procedure for bringing the equipment to a stop.
In conclusion, the switching power supply in accordance with the foregoing first prior art example wherein a power failure is detected in the AC circuit has advantages described below.
The time length for detecting a power failure is extremely short. Since the time length (t.sub.3 -t.sub.1) is determined by the electrostatic capacity of the capacitor 1 which is arranged between the rectifier 40 and the switching means 2, it is not necessarily difficult to make the time length wherein the DC output voltage V.sub.O is held, long enough.
Incidentally, however, the switching power supply in accordance with the foregoing first prior art example is accompanied by disadvantages described below.
Since a power failure is detected in the AC circuit, the means for detecting a power failure 10 readily detects an instanteneous voltage drop or a voltage oscillation in which the voltage which once decreased is promptly recovered, despite this instanteneous voltage drop should not be detected. Therefore, unnecessary interruption is unavoidable for NC equipment or the like supplied with electric power by the switching power supply of the foregoing first prior art example.
Further, the switching power supply of the foregoing first prior art example has a tendency to erroneously identify a distortion in the wave form of an AC voltage as a power failure. Therefore, also in this sense, the switching power supply of the foregoing first prior art example readily causes unnecessary interruption of NC equipment or the like supplied with electric power thereby.
Therefore, the switching power supply in accordance with the foregoing first prior art example is involved with a drawback wherein the reliability is not necessarily satisfactory.
This second example of a prior art switching power supply having the ability to continue supplying electric power after a power failure will now be discussed.
This switching power supply includes a switching circuit to intermit the DC current, a pulse width modulation means and a means for increasing the predetermined allowable maximum value of the duty ratio, when necessary. Detection of a power failure is conducted in the DC circuit.
Referring to FIG. 3, the switching power supply in accordance with the second example available in the prior art is provided with plural members identical to those which are symbolized 1-9 in the first prior art example. Since detection of a power failure is conducted in the DC circuit, this switching power supply accepts not only an AC input power supply but also a DC input power supply. Therefore, the rectifier 40 is illustrated in a broken line.
The switching power supply is further provided with a second voltage comparator 11 which compares the DC output voltage V.sub.O and a second reference voltage V.sub.RR which is predetermined to be less than the first reference voltage V.sub.R and with a pulse width modulation means working voltage range expansion means 12 which outputs a command to expand the maximum value to which the period .DELTA.T in which the switching means 2 is closed, is allowed to increase. The value of the second reference voltage V.sub.RR which is selected to be less than the first reference voltage V.sub.R should not be so much less, otherwise the time length wherein the DC output voltage V.sub.O is held can not be long enough. The pulse width modulation means working voltage range expansion means 12 has a function to expand the maximum length of time .DELTA.T.sub.max until the duration period in which the switching means 2 is closed is allowed to be increased beyond a predetermined value, in response to the output signal .DELTA.V.sub.1 of the second voltage comparator 11. Incidentally, since the second voltage comparator 11 also has a function for detecting a power failure, the output signal .DELTA.V.sub.1 of the second voltage comparator 11 is inputted also to the NC equipment or the like.
Therefore, the NC equipment or the like commences a sequential procedure to stop the equipment without being accompanied by a malfunction, at the time illustrated by t.sub.1 in FIG. 4 (c) at which the second voltage comparator 11 outputs a power failure signal .DELTA.V.sub.1.
Referring to FIG. 4 and the time chart, shown described below will be the behavior of the switching power supply in accordance with the foregoing second prior art example in case of a power failure.
As is illustrated in FIG. 4 (a), an input AC or DC power supply is supposed to be suspended at the time t.sub.0. As is illustrated in FIG. 4 (b), the voltage of the capacitor 1 gradually decreases. As a result, referring to FIG. 4 (c), the DC output voltage V.sub.O begins to decrease. Since the pulse width modulation means 9 functions, however, the duration period in which the switching means 2 is closed is increased, and the DC output voltage V.sub.O is held at the reference voltage V.sub.R until the duration period in which the switching means 2 is closed arrives at the predetermined maximum value. After the duration period in which the switching means 2 is closed expires, the DC output voltage V.sub.O begins to decrease, and it continues decreasing beyond the acceptable minimum voltage (the minimum voltage with which NC equipment or the like is allowed to stop the function thereof without being accompanied by any malfunction) at the time t.sub.3 illustrated in FIG. 4 (c). It is not sure if NC equipment or the like is able to continue the normal function after the time t.sub.3.
The output signal .DELTA.V.sub.1 which the foregoing second voltage comparator 11 outputs at the time t.sub.1 is inputted to NC equipment or the like as a power failure signal.
Therefore, the time length (t.sub.3 -t.sub.1) between the time t.sub.1 at which the second voltage comparator 11 functions and the time t.sub.3 at which the DC output voltage V.sub.O has decreased to the minimum voltage with which the NC equipment or the like is allowed to stop the function thereof without being accompanied by a malfunction, is the time length wherein the DC output voltage V.sub.O is held, for this example. Thus, this time length wherein the DC output voltage V.sub.O is held is required to be longer than the time length required for the NC equipment or the like to finish the procedure for bringing the equipment to a stop.
The electrostatic capacity of the capacitor 1 included in the switching power supply is required to be selected to be large enough to satisfy the foregoing requirements.
The foregoing description has clarified that the switching power supply in accordance with the foregoing second prior art example has advantages and disadvantages described below.
Since detection of a power failure is conducted for the DC output voltage V.sub.O, the switching power supply is insensitive to an instanteneous voltage drop or an AC voltage oscillation or to distortion in the wave form of an AC voltage, the switching power supply is free from an unexpected or unnecessary interruption of NC equipment or the like which is supplied with electric power by the switching power supply.
Since detection of a power failure is not so fast as the switching power supply in accordance with the foregoing first prior art example, this switching power supply is involved with a drawback wherein the electrostatic capacity of the capacitor 1 can not be fully utilized to prolong the time length for detecting a power failure. In other words, because of the foregoing reasons, the time length for detecting a power failure or the length of time which NC equipment or the like is allowed to use for the sequential procedure to stop the function thereof without being accompanied by a malfunction can not be so long.